The present invention relates to an exhaust gas purifying apparatus for purifying exhaust gas of an internal combustion engine.
In such an exhaust gas purifying apparatus, a catalyst arranged in an exhaust path of the engine purifies exhaust gas. For example, Japanese Laid-Open Patent Publication No. 11-294146 discloses measures for restoring the function of the catalyst for NOx (nitrogen oxides) when the catalyst is poisoned by HC (hydrocarbon). In the case of the above publication, NOx in the exhaust gas is purified by reacting with HC as the exhaust gas flows through the catalyst. For this reason, in a case of a diesel engine, since HC contained in exhaust gas is insufficient, fuel, which is light oil, is added to the catalyst to supplement the shortage of HC. However, when the light oil is added for a long period of time, the light oil that has not vaporized is absorbed by the active site of the catalyst. As a result, the catalytic activity could be significantly reduced.
The above publication discloses a method for heating the catalyst to 450° C. or more while stopping some of cylinders to restore the poisoned catalyst to an original state.
As described above, the light oil needs to be supplied to the catalyst after being sprayed into the exhaust path and vaporized. However, if the light oil is supplied to the catalyst without being sufficiently vaporized, the light oil easily adheres to the catalyst, which increases the possibility of poisoning the catalyst. In the prior art, measures for restoring the poisoned catalyst to the original state have been proposed. However, measures for preventing the catalyst from being poisoned by sufficiently vaporizing an additive have not been proposed.